Cable accessories, such as cable splices (or joints) and terminations, are used in a wide variety of medium and high voltage electrical applications, with “medium voltage” representing voltage levels of at least 5 kV Um (where Um is the maximum system voltage) and “high voltage” representing voltage levels of at least 72.5 kV Um. High-voltage cables normally comprise a ground layer covering the cable insulation around the central conductor. The ground layer is often semiconductive or conductive, and is electrically connected to ground potential at some distance from the terminated cable end. Where a cable is to be terminated or joined with another cable, the insulation is normally cut back to expose the central conductor, and the ground layer is cut back further so that its edge is located at a distance from the end of the insulation. The electric field lines between the central conductor and the ground layer concentrates at the edge of the ground layer close to the terminated cable end. On the one hand, a greater distance between the exposed part of the central conductor and the edge of ground layer reduces the risk of electrical breakdown in the strong electric field between the central conductor and the edge of the ground layer. On the other hand, that distance should be kept to a minimum in order to keep the size of a termination or joint small so that their cost is lower and their installation is easier.
A termination or joint of a high-voltage cable must manage the strong electrical field between the central conductor and the edge of the ground layer at the end portion of the cable in order to avoid high electric “stress”, i.e. negative effects caused by partial discharges, which may result in long-term electrical breakdown or electrical erosion, and interface discharges. These discharges are caused by the strong electric field on the materials of the cable and those of the termination or joint. In particular, it is desirable to avoid an extreme concentration of the electric field lines at the edge of the ground layer, while keeping the physical size of the entire termination or joint within reasonable limits.
Different field control or stress control approaches have been taken to avoid concentration of the electric field, like resistive stress control, refractive stress control, capacitive stress control or geometric stress control. Traditionally, many cable termination devices or joint devices had stress cones, which provided a conductive ground layer on the outer surface of a cone-shaped element.
In particular, terminations for such applications have traditionally utilized wet or oil filled devices, with a conductive or semi-conductive geometric stress cone to grade the electric stress and with a dielectric oil provided for insulation. These wet terminations can be complicated and can develop leaks over time, leading to failure of the device.
Alternatively, dry terminations have been used with a similar geometric stress control and a polymeric insulation. These dry type terminations typically require longer cable cut backs and can be difficult to install in a push-on application. Another type of termination is a dry GIS type or plug-in termination. These GIS terminations can be complicated to install and can cost much more than a traditional termination.
It is desirable to improve the stress control properties of cable terminations and joints in order to reduce the risk of electrical breakdown and damage to the cables or to the terminations or joints. In particular, it is desirable to provide stress control means that provide reliable stress control at higher voltages, without requiring more space. The present invention addresses these needs.